Field of the Invention
Acousto-optical device scanning techniques and systems that suppress optical crosstalk are described.
Related Art
During semiconductor fabrication, isolated and/or systemic defects may be formed on the wafer. Isolated defects, which are present in a low percentage of chips on the wafer, may be caused by random events such as an increase in particulate contamination in a manufacturing environment or an increase in contamination in the process chemicals used in the fabrication of the chips. Systemic defects, which are typically present in a high percentage of chips on the wafer, may be caused by defects on a reticle. A reticle is used to transfer a pattern for an integrated circuit layer onto the wafer using photolithographic techniques. Therefore, any defect on the reticle may be transferred with the pattern to each chip of the wafer.
Automated inspection systems have been developed to inspect a wafer surface (both unpatterned and patterned). An inspection system typically includes an illumination system and a detection system. The illumination system may include a light source (e.g. a laser) for generating a beam of light and an apparatus for focusing and scanning the beam of light. Defects present on the wafer surface may scatter the incident light provided by the illumination system (also called an illuminator). The detection system is configured to detect the scattered light and convert the detected light into electrical signals that can be measured, counted, and displayed. The detected signals may be analyzed by a computer program to locate and identify defects on the wafer. Exemplary inspection systems are described in U.S. Pat. No. 4,391,524, issued to Steigmeier et al. on Jul. 5, 1983, U.S. Pat. No. 4,441,124, issued to Heebner et al. on Apr. 3, 1984, U.S. Pat. No. 4,614,427, issued to Koizumi et al. on Sep. 30, 1986), U.S. Pat. No. 4,889,998, issued to Hayano et al. on Dec. 26, 1989, and U.S. Pat. No. 5,317,380, issued to Allemand on May 31, 1994, all of which are incorporated by reference herein.
One or more components used in a state-of-the-art illumination system may use acousto-optics. For example, FIG. 1A illustrates a simplified configuration of an acousto-optical device (AOD) 100. AOD 100 includes a sound transducer 121, a quartz plate 122, and an acoustic absorber 123. An oscillating electric signal can drive sound transducer 121 and causes it to vibrate. In turn, this vibration creates sound waves in quartz plate 122. Acoustic absorber 123 is configured to absorb any sound waves that reach the edge of quartz plate 122. As a result of the sound waves, incoming light 124 to quartz plate 122 is diffracted into a plurality of directions 128, 129 and 130.
A diffracted beam emerges from quartz plate 122 at an angle that depends on the wavelength of the light relative to the wavelength of the sound. By ramping frequencies from high to low, portion 126 may have a higher frequency than portion 127. Because portion 126 has a higher frequency, it diffracts a portion of the incident light beam through a steeper angle as shown by diffracted beam 128. Because portion 127 has a relatively lower frequency, it diffracts a portion of the incident light beam through a more shallow angle as shown by diffracted light beam 130. Because a mid-section portion between portions 126 and 127 has a frequency between the higher and relatively lower frequencies, it diffracts a portion of the incident light beam through an intermediate angle as shown by diffracted light beam 129. This is an example of how an AOD can be used to focus an incoming beam 124 at position 125.
Notably, AODs can operate significantly faster than mechanical devices, such as mirrors. Specifically, AODs can diffract incoming light in approximately the time it takes the sound wave to cross the incoming light beam (e.g. 5-100 ns). Thus, a scan of a sample, e.g. of a wafer or reticle, can be performed at a rate of, for example, 6.32 mm/μsec.
FIG. 1B illustrates an exemplary dual AOD illumination system 110 configured to generate and scan a beam across a sample 109, such as a wafer. A prescan AOD 101 is used to deflect the incident light from a light source 100 at an angle, wherein the angle is proportional to the frequency of the radio frequency (RF) drive source. A telephoto lens 102 is used to convert the angular scan from prescan AOD 101 into a linear scan.
A chirp AOD 104 is used to focus the incident beam in the plane of acoustic propagation onto a scan plane 105. This is accomplished by ramping thru all the RF frequencies with transducer 104A faster than those frequencies can all propagate thru chirp AOD 104. This rapid ramping forms a chirp packet 104B. Chirp packet 104B then propagates thru chirp AOD 104 at the speed of sound. FIG. 1B shows the location of chirp packet 104B at the start of a spot sweep, whereas FIG. 1C illustrates the location of chip packet 104B at the end of that spot sweep. Note that during this propagation, prescan AOD 101 adjusts its RF frequency to track the chirp packet in AOD 104 to keep the light beam incident upon chirp packet 104B.
A cylinder lens 103 is used to focus the beam in a plane perpendicular to the plane of acoustic propagation. A relay lens 106 is used to generate a real pupil at a pupil plane 106A. A magnification changer 107 is used to adjust the size of the spot and the length of sweep. An objective lens 108 is used to focus the spot onto a sample 109, such as a wafer.
FIG. 2 illustrates another exemplary illumination system 200 using a single AOD. In system 200, the prescan AOD is replaced by a beam expander 201. Therefore, this type of illumination system is called a “flood AOD” system. In this configuration, multiple chirp packets 203A and 203B are generated in AOD 104. Note that components having the same numerical references herein are substantially similar components and therefore their descriptions are not repeated. Each chirp packet 203A and 203B generates its own spot. Therefore, objective lens 108 focuses two spots onto sample 109 simultaneously. Although two chip packets are shown in FIG. 2, in other embodiments, additional chirp packets may be generated with a corresponding number of spots incident on sample 109.
Note that sample 109 is typically placed on an XY translation stage capable of bi-directional movement. In this configuration, the stage can be moved so that the focused spots (formed by the focusing optics using the diffracted light beams) impinging sample 109 can be scanned along adjacent contiguous strips of equal width (i.e. raster scan lines). U.S. Pat. No. 4,912,487, issued to Porter et al. on Mar. 27, 1990, and incorporated by reference herein, describes exemplary illumination systems including a translation stage configured to provide raster scanning.
FIG. 3 illustrates a known exemplary AOD scanning technique providing isolation of scattered light for multiple spots. In this embodiment, four spots are scanned during four times 301, 302, 303, and 304 (each spot having a same fill pattern for ease of reference in FIG. 3). These four spots can be generated by an illumination system including an AOD. In FIG. 3, the AOD provides a chirp packet spacing 306 (which also correlates to the spot spacing and the scan line segment length).
FIG. 4 illustrates an exemplary inspection system 400 for the technique described in FIG. 3. In system 400, an AOD optical path, e.g. similar to that shown in FIG. 2, can include an objective lens 404 for focusing the spots generated by the AOD onto a sample 401. System 400 further includes a 50/50 beam splitter (or other ratio) 405 that can direct two copies of the scattered light 402 from the scanned spots on sample 401 to two detector arrays 408 and 409. A first collection path and mask set 406 can be configured to isolate the scattered light from a first set of spots and provide its output to detector array 408, whereas a second collection path and mask set 407 can be configured to isolate the scattered light from a second set of spots and provide its output to detector array 408. Note that each mask has a set of windows, each window having a predetermined width for a given PMT (photomultiplier tube) or other sensor.
Referring back to FIG. 3, the first set of spots is indicated by the boxes having solid lines, whereas a second set of spots is indicated by the boxes having dotted lines. The length of the boxes corresponds to a window width 305 used for the masks in FIG. 4. Thus, for example, at time 301, the scattered light from spots 310 and 312 (using collection path and mask set 406) can be isolated from spot 311 (using collection path and mask set 407). To ensure complete coverage, a mask overlap 307 is provided.
In the scanning technique of FIG. 3, two requirements must be met. First, PMT window width 305 must be smaller than the desired line segment length, which is the spacing of the AOD chirp packet as shown by 306. Second, the PMT windows must overlap, as shown by overlap 307, but must not extend beyond the desired segment length. This requirement ensures that only one spot is within a given mask at any time. Assuming both requirements are met, the scanning technique of FIG. 3 can provide appropriate isolation for the scattered light because at no time are there two spots in a single box.
However, this mask overlap can sometimes result in both arrays of detectors capturing the scattered light from the same spot, as shown by spot 313 at time 301. A similar condition occurs during time 304 for spot 314. This duplicated information must be recognized and accounted for during analysis, thereby increasing collection system complexity. Note also that sometimes a spot is not within the area designated for a mask, as shown by area 315 for time 302 and area 316 for time 303. In those cases, information must still be captured even though no spot is present, thereby wasting resources.
Moreover, 50/50 beam splitter 405 undesirably reduces the light available for detection by one-half. To overcome this disadvantage, a laser (light source) that is 2× higher power would be needed, thereby increasing the cost of the inspection system. Assuming the maximum power laser is already being used, an inspection system using a 50/50 beam splitter would require a large laser. Having multiple chirp packets in the AOD simultaneously as shown in FIG. 2 would have high spot-to-spot crosstalk because of the relatively close proximity of the spots to one another. Moreover, because the PMT window is smaller than the desired line segment length more PMTs are needed, thereby yet further increasing inspection system cost.
FIG. 5A illustrates another exemplary AOD illumination system 500 that can generate multiple spots without flood illumination. In this embodiment, a diffractive optical element (DOE) 501 can be positioned before magnifier changer 107 to generate a plurality of spots. Although FIG. 5A shows three spots being generated (different line colors indicating different beams associated with those spots), other embodiments can generate a different number of spots. FIG. 5B illustrates the effects of changing the magnification of magnifier changer 107 on the spot size, spot spacing, and scan length on sample 109 for illumination system 500. Note that the different fill colors indicate different spots (and correspond to the different line colors of FIG. 5A). As shown in FIG. 5B, large spots 520 have spacing associated with three positions 1, 3, and 5, whereas small spots 521 have spacing associated with three positions 2, 3, and 4. The large spot in position 1 scans to position 3, the large spot in position 3 scans to position 5, and the large spot in position 5 scans to position 7. In contrast, the small spot in position 2 scans to position 3, the small spot in position 3 scans to position 4, and the small spot in position 4 scans to position 5.
Having a smaller spot size (higher magnification), makes appropriate isolation for the scattered light from the multiple spots more difficult. For example, FIGS. 6A and 6B illustrate exemplary sweeps of three small spots 601, 602, and 603 (corresponding to those shown in FIG. 5B) between times T1 and T4. FIG. 6B represents the scans of spots 601, 602, and 603 as boxes of the same color, wherein the boxes represent the paths of the spots as a result of the propagation through the chirp AOD. FIG. 6B shows that there is an overlap of the co-linear scans of different spots (which would occur for both the big spots and the small spots). This overlap will result in undesirable spot crosstalk.
To provide the appropriate isolation between spots, thereby minimizing crosstalk, additional optics and techniques are required. In one embodiment, shown in FIGS. 7A and 7B, a prism 705 can be used in an illumination system to create the appropriate spacing between the spots. U.S. Pat. No. 7,075,638, issued to Kvamme on Jul. 11, 2006, and incorporated by reference herein, describes such an illumination system. In this system, prism 705 and additional optics, such as a spherical aberration correction lens and a transmitted lens, are positioned such that scattered light from the plurality of spots, e.g. beams associated with spots 701, 702, and 703, on the sample are directed to a specific facet of prism 705, as shown in FIG. 7A. In turn, prism 705 directs each beam to a separate detector. FIG. 7B shows the scan sweeps of spots 701, 702, and 703 during operation of the associated inspection system. Prism 705 (which is part of the collector) takes advantage of an offset shown in FIGS. 7A and 7B (the offset being generated by a grating, which is part of the illumination system) to desirably increase the spot isolation. Thus, referring back to FIG. 6B, turning a grating will result in spots 701, 702, and 703 (and their associated scans) no longer being co-linear along the x-axis (i.e. they will instead form a diagonal line with offset scans in a horizontal plane). Unfortunately, prism 705 is designed for a specific magnification. Therefore, if the magnification is changed, then another prism must be used, thereby adding cost and design complexity to the inspection system.
The accurate detection of defects on a sample surface depends on the correct measurement and analysis of each spot in the scan. Therefore, a need arises for optimizing techniques and systems using AODs that ensure the isolation of these spots, thereby minimizing crosstalk, while minimizing system complexity and cost.